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United States Patent |
5,773,379
|
Bonifay
,   et al.
|
June 30, 1998
|
Thermal regulation process for a solid in a heat exchanger using
cylindrical tube surfaces
Abstract
The invention concerns a process for thermal regulation in a continuous
fluidized bed treatment process for a powdered solid, wherein the solid is
treated in a fluidized bed treatment zone, at least a portion of the solid
is extracted from said zone and transported to an external heat exchanger
(21) containing at least one array (22) of thermal exchange tubes in which
vaporizable cooling fluid circulates, fluidized or mobile bed thermal
regulation by indirect heat exchange with the fluid is carried out and the
portion of regulated solid is extracted for recycling into the treatment
zone or to another treatment zone (1). More precisely, said portion of
solid is circulated in descending mode by means of an inert or non inert
fluidization fluid across the array of tubes (22) which are wound such
that the current of solid intersects said tubes and that the cooling fluid
is circulated in one direction in the array of tubes.
Inventors:
|
Bonifay; Regis (Asnieres, FR);
Gauthier; Thierry (Saint Genis Laval, FR);
Pontier; Renaud (Vienne, FR);
Hoffmann; Frederic (Paris, FR)
|
Assignee:
|
Institut Francais Du Petrole (Rueil Malmaison, FR)
|
Appl. No.:
|
545870 |
Filed:
|
November 13, 1995 |
PCT Filed:
|
May 4, 1994
|
PCT NO:
|
PCT/FR94/00522
|
371 Date:
|
November 13, 1995
|
102(e) Date:
|
November 13, 1995
|
PCT PUB.NO.:
|
WO94/26845 |
PCT PUB. Date:
|
November 24, 1994 |
Foreign Application Priority Data
Current U.S. Class: |
502/44; 208/113; 208/160; 502/41 |
Intern'l Class: |
B01J 020/34 |
Field of Search: |
502/44,41
208/160,113
|
References Cited
U.S. Patent Documents
2963422 | Dec., 1960 | Hann | 208/160.
|
4422302 | Dec., 1983 | Davis et al. | 62/57.
|
4434245 | Feb., 1984 | Lomas et al. | 502/2.
|
4439533 | Mar., 1984 | Lomas et al. | 502/41.
|
4471063 | Sep., 1984 | Hettinger | 502/40.
|
4753907 | Jun., 1988 | Zandona et al. | 502/44.
|
4757039 | Jul., 1988 | Lomas | 502/44.
|
4964523 | Oct., 1990 | Mauleon et al. | 502/44.
|
5120691 | Jun., 1992 | Pontier et al. | 502/44.
|
5286690 | Feb., 1994 | Hoffmann et al. | 502/44.
|
Foreign Patent Documents |
0092065 | Oct., 1983 | EP.
| |
0153214 | Aug., 1985 | EP.
| |
0192906 | Sep., 1986 | EP.
| |
0197486 | Oct., 1986 | EP.
| |
0403381 | Dec., 1990 | EP.
| |
0093063 | Nov., 1993 | EP.
| |
2015263 | Apr., 1970 | FR.
| |
2124043 | Sep., 1972 | FR.
| |
3305471 | Aug., 1983 | DE.
| |
Primary Examiner: Griffin; Walter D.
Attorney, Agent or Firm: Millen, White, Zelano, & Branigan, P.C.
Claims
We claim:
1. A process for thermal regulation or control in a continuous fluidized
bed regeneration process for a used catalyst containing coke,
comprising regenerating the catalyst in at least one dense fluidized bed
regeneration zone;
extracting at least a portion of said catalyst from said zone;
transporting said portion of catalyst in the regeneration zone and a
portion of regeneration fumes by a downwardly inclined conduit to an
external thermal exchange zone, having an axis of symmetry, an upper
extremity and a lower extremity, and containing at least one thermal
exchange tube array wherein a vaporizable cooling fluid circulates, said
conduit connecting the dense bed of the regeneration zone to the thermal
exchange zone and opening thereinto at a junction point located under the
level of the dense bed of the regeneration zone and so disposed to provide
a dense bed zone of catalyst bounded by the lower extremity of the thermal
exchange zone to above the junction point, the upper level of said zone
being established substantially to the level of the catalyst in the
regeneration zone;
providing a release zone open volume above said dense bed to the upper
extremity of the exchange zone;
cooling the catalyst in said dense bed thermal exchange zone under indirect
thermal exchange conditions and with fluidization, in the presence of a
fluidization gas;
circulating the catalyst downwards counter-currently to the fluidization
gas flow direction, and separating the catalyst from the fluidization gas
and any regeneration fumes in said release zone volume;
evacuating said gases and fumes from the release zone and transporting the
resultant evacuated gases to a dilute phase above the dense bed in the
regeneration zone; and recycling the resultant cooled catalyst from the
lower portion of the thermal exchange zone to the regeneration zone,
the process being characterized in that said tube array is wound or
disposed so that the portion of catalyst which is circulated across the
tube array in the fluidized bed in a descending mode by means of the
fluidization gas intersects said tubes over the cross section of the
exchange zone, the lower end of said tube array being connected to an
axial conduit for transporting the cooling fluid and maintaining said
array.
2. A process according to claim 1 wherein said array comprises tubes wound
into a helix with a plurality of helix diameters with respect to a winding
axis which is substantially parallel to the axis of symmetry of the
exchange zone, such that tubes having the same winding diameter form a
cylindrical layer or surface and that the different cylindrical layers or
surfaces thus formed are disposed one inside the other.
3. A process according to claim 1, wherein the exchange zone comprises a
plurality of tubular arrays comprising a first series of substantially
parallel tube sections whose axes are located in a first plane and a
second series of substantially parallel tube sections whose axes are
located in a second plane which is substantially parallel to the first
plane, the sections in one plane being at a substantially equal and
opposite inclination to that of the sections in the other plane with
respect to the longitudinal plane containing said axis of symmetry, the
tube sections with ends adjacent to one side being connected together by
bent sections, the assembly of tubular arrays being positioned
substantially parallel to the plane containing the axis of symmetry of the
thermal regulation zone.
4. A process according to claim 1, wherein the exchange zone comprises a
plurality of tube arrays wherein each tube comprises tube sections which
are substantially parallel to each other and located in the same plane,
the ends adjacent to one side of two successive sections being connected
together by bent sections, the assembly of tube arrays being positioned
substantially parallel to the plane containing the axis of symmetry of the
thermal regulation zone.
5. A process according to claim 1, comprising two used-catalyst
regeneration zones, conducting a first catalyst regeneration step in a
first regeneration zone to produce an at least partially regenerated
catalyst, transporting the latter to a second regeneration zone located
above the first zone, conducting a second regeneration step and cooling at
least a portion of the catalyst from the second regeneration zone in
accordance with claim 1 and extracting the cooled catalyst from the
exchange zone for transport to the first regeneration zone or return to
the second regeneration zone.
6. A process according to claim 1, wherein said tubes are in the form off a
helix having an interaxial distance between the tubes of 1.5 to 10 times
the diameter of the tubes.
7. A process according to claim 1, wherein the thermal exchange array
occupies substantially the entire volume of the dense bed in the thermal
exchange zone.
8. A process according to claim 1 wherein the flow rate of the cooling
fluid in the tubes is 0.5 to 2.5 m/s.
9. A process according to claim 1 wherein the cooling fluid circulates in
the tubes from bottom to top.
10. A process according to claim 1, wherein the flow rate of the
fluidisation gas in the thermal exchange zone is 0.01 m/s to 0.75 m/s.
11. A process according to claim 6, wherein the interaxial distance between
the tubes is 2 to 3 times the diameter of the tubes.
12. A process according to claim 10, wherein the flow rate is 0.05 m/s to
0.3 m/s.
13. A process according to claim 1, wherein the exchange zone is an
elongate zone.
14. A process according to claim 1, wherein the fluidization gas intersects
said tubes over substantially the entire cross section of the exchange
zone.
Description
FIELD OF THE INVENTION
The invention concerns the use of an array of specially wound tubes in a
fluidised or mobile bed heat exchanger. It generally concerns a process
for thermal regulation or control in a continuous fluidised or mobile bed
treatment process for powdered solid. The invention particularly concerns
a process for regeneration of a used catalyst by heat exchange in a
fluidised bed. The process is particularly applicable to the regeneration
of catalysts which are particularly heavily loaded with hydrocarbon
residues and coke following reaction with a hydrocarbon feedstock. It can
apply to catalysts for hydrotreatment, hydrocracking or catalytic
cracking, reforming catalysts or to any contact mass used, for example, in
thermal cracking processes.
By way of illustration, we shall describe the use of an array of tubes in a
fluidised bed process for regeneration of a used catalyst from a catalytic
cracking process using heavy feedstocks having a high Conradson carbon
residue such as atmospheric pressure residue, vacuum residue, or
deasphalted residue, these residues being capable of hydrotreatment.
The process is particularly applicable to temperature control.
BACKGROUND OF THE INVENTION
Catalytic cracking processes convert hydrocarbon feedstocks into lighter
products such as petrol. The feedstocks were initially fairly light, for
example gas oils, and in order to achieve maximum conversion efficiency
using highly active zeolite catalysts, it was necessary to remove the
maximum amount of coke which deposited on these catalysts and reduced
their activity. This was carried out in a regeneration step at a
temperature of between 520.degree. C. and 800.degree. C.
Increasing demand for fuels led refiners to use heavier and heavier
feedstocks, containing high boiling point hydrocarbons, with boiling
points of more than 550.degree. C., for example, and a high Conradson
carbon residue or high concentration of metals. Large quantities of coke
and heavy hydrocarbons can become deposited on the catalyst during the
catalytic cracking phase. Combustion regeneration releases a large amount
of heat which can cause the apparatus to deteriorate and deactivate the
catalyst, particularly when exposed to temperatures of greater than
800.degree. C. for long periods. It is therefore essential to control
catalyst regeneration. This problem is particularly prominent when
existing technology essentially designed for the treatment of conventional
feedstocks is employed in a process using much heavier feedstocks.
The following patent documents illustrate the state of the art: EP-A-0 092
065, U.S. Pat. No. 2,963,422, EP-A-0 197 486, EP-A-0 153 214, EP-A-192 906
and EP-A-0 093 063. In addition, European patent EP-A-0 403 381 describes
a double chamber heat exchanger connected to a catalyst regenerator by a
single entrance and exit opening, which limits the cooling efficiency of
the exchanger.
The prior art is further illustrated by U.S. Pat. No. 4,434,245 which
describes thermal exchange of a catalyst which has been removed from a
release zone located above a combustion zone and comprising vertical
bayonet tubes. The drawback here is that a limiting layer of defluidised
catalyst located at the tube walls flows along the tube and reduces
thermal exchange.
Complex and cumbersome technology is usually required to overcome this
problem, thereby reducing the reliability of the system.
A horizontal tubular plate positioned in the thermal exchanger and
supporting the tubes will maintain the mechanical integrity of the
assembly. However, widely different temperature and pressure conditions in
different parts of the plate, for example 730.degree. C., 3 bars on the
catalyst side and 275.degree. C., 60 bars on the coolant water side,
create intense stresses which affect the mechanical integrity of the
assembly.
SUMMARY OF THE INVENTION
One object of the invention is to overcome the problems associated with the
prior art and obtain maximal thermal exchange in the most homogeneous
fashion.
The invention thus concerns a process for thermal regulation or control in
a continuous fluidised or mobile bed treatment process for a powdered
solid, wherein the solid is treated in a fluidised or mobile bed treatment
zone, at least a portion of the solid is extracted from said zone and
transported to an external thermal regulation or control zone which is
advantageously elongate and has an axis of symmetry and which contains at
least one array of thermal exchange tubes in which vaporisable cooling
fluid circulates, fluidised or mobile bed thermal regulation by indirect
heat exchange with the fluid is carried out and the regulated portion of
solid is extracted for recycling into said treatment zone or to another
treatment zone. More precisely, said portion of solid is circulated in
descending mode by means of an inert or non inert fluidisation fluid
across the array of tubes which are wound or disposed such that the
current of solid intersects said tubes, preferably across substantially
the entire cross section of the regulation zone, and such that the cooling
fluid is circulated in one direction in the array of tubes.
In a first preferred embodiment of the process, the thermal exchange array
comprises tubes wound into a helix with a plurality of diameters with
respect to a winding axis which is substantially parallel to the axis of
symmetry of the regulation zone, such that tubes having the same winding
diameter form a cylindrical layer or surface and that the different layers
or surfaces thus formed are disposed one inside the other, preferably
substantially concentrically.
This configuration is described in French patent FR-A-2 124 043 as applied
to steam generators which are reheated with liquid sodium. Within the
context of the invention, this configuration produces maximum exchange in
the fluidised bed since substantially the entire volume of the cylindrical
thermal exchanger containing the dense phase solid is cooled by the array
of tubes.
In a second embodiment, the thermal regulation zone comprises a plurality
of tubular arrays comprising a first series of substantially parallel tube
sections whose axes are located in a first plane and a second series of
substantially parallel tube sections whose axes are located in a second
plane which is substantially parallel to the first plane, the sections in
one plane being at a substantially equal and opposite inclination to that
of the sections in the other plane with respect to the longitudinal plane
containing said axis of symmetry, the tube sections with adjacent ends on
one side being connected together by bent sections, the assembly of
tubular arrays being positioned substantially parallel to the plane
containing the axis of symmetry of the thermal regulation zone.
Such a configuration is described in French patent FR-A 2 015 263.
In a third embodiment, the thermal regulation zone comprises a plurality of
tube arrays wherein each tube comprises tube sections which are
substantially parallel to each other and located in the same plane, the
ends adjacent to one side of two successive sections being connected
together by bent sections, the assembly of tube arrays being positioned
substantially parallel to the plane containing the axis of symmetry of the
thermal regulation zone.
These tube arrays may advantageously be used in a process for continuous
fluidised bed regeneration of a used catalyst by combustion of the coke
deposited on the catalyst during the course of a hydrocarbon conversion
reaction in a reaction zone. The process thus comprises at least one
regeneration zone into which said catalyst is introduced from said
reaction zone, the catalyst is normally regenerated in a dense fluidised
bed zone in the presence of a gas containing oxygen under regeneration
conditions, at least a portion of the catalyst is extracted from the dense
fluidised bed and transported to the thermal regulation or exchange zone,
said portion of catalyst is cooled by indirect heat exchange with the
cooling fluid, and the cooled portion of catalyst is reintroduced into the
dense fluidised bed in the regeneration zone.
In accordance with one mode of operation, the cooled catalyst in the lower
portion of the thermal exchange zone is recycled to the dense bed of the
generator from which it was extracted by means of co-current injection of
fluidisation gas containing oxygen. The cooled catalyst is thus circulated
to the regenerator in broadly ascending mode using recycling means
comprising a catalyst evacuation conduit controlled by a valve connected
to a Y or J junction, for example, which is itself connected to a catalyst
gas lift which has fluidisation air injected at its base.
A further mode of operation employing an array is used in a continuous
fluidised bed regeneration process for a used catalyst by combustion of
coke deposited thereon, comprising two regeneration zones. A first
catalyst regeneration step is carried out in a first regeneration zone,
the at least partially regenerated catalyst is transported to a second
regeneration zone located above the first zone, a second regeneration step
is carried out and at least a portion of the catalyst from the second
regeneration zone is cooled under the conditions described above and the
cooled catalyst is extracted for transport to the first regeneration zone
or return to the second regeneration zone.
If the catalyst is recycled to the first regeneration zone, it can be
gravity fed (descending mode).
When the catalyst is recycled to the first regeneration zone, it can be
recycled in generally ascending mode using the recycling means described
above comprising the gas lift.
When recycling to the second regeneration zone, the catalyst can be
recycled using the recycling means described above comprising the gas
lift.
In a further mode of operating an apparatus comprising two regeneration
zones, the first catalyst regeneration step is carried out in the first
regeneration zone, at least a portion of the catalyst from the first
regeneration zone is cooled in a thermal exchanger in accordance with the
invention and the cooled catalyst is recycled to the first regeneration
zone using the recycling means described above comprising the gas lift.
The catalyst is then transported from the first regeneration zone to the
second which is located above the first zone.
Whatever the winding of the tube array described above, the interaxial
distance between the tubes defining the helical pitch is between 1.5 and
10 times their diameter, preferably between 2 and 3 times their diameter.
This distance can be that measured between tubes located in the same
cylindrical surface or between two neighbouring cylindrical surfaces (in
the case of the first embodiment). This distance may also be that measured
for tubes positioned in accordance with the second and third embodiments.
Good thermal exchange is obtained with a fluidisation gas flow rate of
generally 0.01 m/s to 0.75 m/s, preferably 0.05 m/s to 0.3 m/s in the
thermal exchange zone using a tube array wherein the cooling fluid and the
vapour generated preferably circulates from the bottom to the top at a
flow rate normally between 0.5 and 2.5 m/s, for example, preferably 1 to 2
m/s, ie counter-current to the flow of catalyst in the thermal exchange
zone.
The catalyst current intersecting the tubes is permanently renewed at the
tube surface within its descending current.
High compactness (for example 13 to 16 m.sup.2 /m.sup.3 of exchanger) and
ease of installation or removal are particular advantages.
Further, the tube array can advantageously absorb radial and axial
expansions.
Finally, because of the high exchange efficiency, the system requires an
installed surface area per unit volume which is less than that of the
prior art. This has the advantage of leaving a greater volume for the
circulating catalyst, thus retaining good fluidisation without the need
for auxiliary means.
The cooling fluid circulating in the exchanger may be air, water, water
vapour or mixtures of these fluids.
The catalyst which has been regenerated in accordance with the invention is
also of conventional type, such as zeolite or amorphous type
aluminosilicates, advantageously with a granulometry of 30 to 100
micrometres.
BRIEF DESCRIPTION OF DRAWINGS
The invention will be better understood from the description of the FIGURE,
an elevational view, which illustrates the process and apparatus.
DETAILED DESCRIPTION
A first regenerator 1 in a catalytic cracking unit receives catalyst on
which coke has been deposited during the course of the catalytic cracking
reaction from a stripper (not shown) via line 2. This line opens into the
catalytic bed at an appropriate point, preferably into the diluted phase
located above dense fluidised bed 3. Regeneration gas containing oxygen is
fed via line 4 to fluidisation means 5, for example a screen, ring or
distribution line, situated at the base of the regenerator to fluidise the
dense catalyst bed and effect continuous counter-current combustion of
about 50% to 90% of the coke. Regeneration fumes and entrained catalyst
are separated in cyclones 6 and the regeneration fumes containing the
major combustion products carbon monoxide, carbon dioxide and water vapour
are evacuated via line 7 to an incinerator.
The temperature of fluidised bed 3 is measured by means of sensor 8. When
this temperature falls below a set value T.sub.1, due to introduction of
the relatively cold catalyst via lines 34 as will be described below, the
flow of oxidising fluid (fluidisation fluid) to fluidisation means 5,
regulated by control valve 33 on line 4, is increased until the
temperature measured at 8 returns to the preset value.
The partially regenerated catalyst particles are then transferred to a
second regenerator 9 located above the first regenerator 1 via conduit 10
which is supplied with air via line 11. Diffuser 12, supplied with air via
line 13, is located at the base of the second regenerator. Combustion of
the partially regenerated catalyst is carried out in dense bed 19 whose
upper portion defines level 19a at a height which depends on the level of
aeration.
A portion of the regenerated catalyst is laterally evacuated into buffer
chamber 14. Particle fluidisation in this chamber is normally controlled
by annular diffuser 15 which is fed with fluidisation gas such as air or
an inert gas via line 16. Regenerated catalyst particles are recycled from
chamber 14 via conduit 35 fed by a riser (not shown) in a quantity which
is determined by the opening or closing of a valve. The combustion gases
in the upper portion of the second regenerator are separated from the
catalyst particles by external cyclones 17 and evacuated via line 18 which
is separate from fume evacuation line 7 from the first regeneration step.
A portion of the hot catalyst and a portion of the fumes at a temperature
of 600.degree. to 850.degree. C. are removed from dense bed 19 of the
second regenerator at a point located above air injection means 12 and
gravity fed via downwardly inclined conduit 20, for example at an angle of
30 to 60 degrees with respect to the axis of the exchanger, into heat
exchanger 21 adapted for indirect heat exchange. The exchanger is
vertical, of elongate cylindrical form, and contains a thermal exchange
array which lines the space containing substantially all the dense
catalyst bed, within an envelope. The array is a tubular array composed of
a plurality of tubes 22a, 22b wound in a helix with a plurality of
diameters with respect to the vertical axis of the exchanger envelope.
Layers with the same winding diameter form a cylindrical layer or surface
and the various coaxial cylinders thus defined are located one within the
other. The chamber which is delimited by the envelope contains the
catalyst which is maintained as a dense bed through the tube array by
means of fluidisation means 24 (ring or screen) into which a
counter-current of air is introduced via line 25. The catalyst circulates
from top to bottom through the array, intersecting the tubes across
substantially the entire cross section of the exchanger and giving up its
heat to an appropriate fluid such as pressurised water which is fed via
line 23a. This line feeds the upper extremity of a central cylindrical
conduit 40 which is thermally insulated and which acts as the winding axis
for the helically disposed tubes and also acts as a stiffener and
therefore as a support system. This axial cylindrical conduit, where
practically no thermal exchange occurs, feeds the various cylindrical
tubular surfaces at different distances from its lower end such that the
water-vapour mixture rises counter-current to the catalyst. Thus the
helixes with the greatest winding diameter are fed by the ends of bent
tubes connected to the lowest points of the axial conduit while those with
a smaller diameter are connected higher up.
The upper part of the tubes of the array have ends which are bent
vertically to connect to the tube surfaces of the steam chests or toric
collectors (not shown), resulting in very high mechanical strength. The
water-vapour mixture is evacuated via line 23b connected to the
collectors.
Conduit 20 carrying hot catalyst opens into the exchanger at a junction
point located below level 19a of the dense bed of the second regenerator,
for example at a point located at a distance of a quarter to a third of
the height of the exchanger from upper extremity 26. The catalyst is in a
dense fluidised bed due to fluidisation means 24 (ring) right across the
tube array to a level 19b above the junction point and substantially
identical to level 19a of the dense bed in regenerator 29. Level 19b is in
general a function of the respective fluidisation flow rates in the second
regenerator and in the thermal exchanger and thus of the respective
densities. There may thus be a small difference between the catalyst
levels in the regenerator and in the exchanger.
The height of the exchange array and the parameters which determine its
compactness are generally calculated so that the array occupies
substantially the entire volume of the dense bed in the exchanger.
Preferably, the height is equal to the highest level which the dense bed
in the exchanger can attain, taking into account the differences in
pressure which may exist in the exchange zone and in the regenerator.
The height of the exchanger is selected so that, with respect to the level
in the regenerator, a 1 to 2.5 m high free zone termed the release zone 27
is created in the exchanger above the dense bed to allow the fluidisation
gas and any regeneration fumes to be separated from the catalyst. A
degassing line 28 evacuates the fumes and gases from the dilute phase at
the upper extremity of the exchanger and carries them to the diluted
fluidised phase 29 above the dense fluidised bed of the second
regenerator. The diameter is selected so that the ratio of the diameter of
the degassing line to that of the catalyst inlet conduit 20 is between 3
and 6. The gas exit rate is generally between 3 and 15 m/s.
Extraction and recycling means 34 comprises a conduit in which the catalyst
flows under gravity and which is the first regenerator. The catalyst is
transported by a lift recycled into the dense phase of the first
regenerator, preferably above fluidisation means 5.
Valve 30, for example a slide gate, is located at the exit to exchanger 21
below the lower extremity of the first regenerator and upstream of the
lift. This controls the rate of transfer of catalyst between the
regenerators once the temperature of the regenerated catalyst exceeds the
required preset value.
The FIGURE is shown with a descending catalyst flow at the exit of the
exchanger and with an ascending flow only in the first regenerator. In a
further unillustrated embodiment, the catalyst is introduced directly into
the dense phase of the first regenerator by descending flow.
The rate of catalyst flow through the thermal exchanger is adjusted to
maintain the temperature in the second regenerator and thus maintain the
entry temperature into the reaction zone (riser) at a set temperature
suitable for the feedstock to be cracked in the unit.
Thermal control of the regeneration process is effected by a combination of
the following means:
Automatic control means 31 is connected to valve 30 located on catalyst
evacuation conduit 17 from the exchanger. This means is also connected to
temperature sensor 32 which monitors the local temperature in the dense
bed of second regenerator 9. When the signal transmitted by the sensor
indicates a value which exceeds a preselected value which depends on the
regeneration parameters and is stored in the automatic control means, a
signal is transmitted by the latter to valve 30 which increases the
catalyst evacuation flow rate and thus increases the catalyst flow rate
into the exchanger. This increase in flow rate helps to reduce the
temperature of the first regeneration step as recorded by temperature
sensor 8. Means 31 compensates by increasing the oxygen feed by adjusting
valve 33 on line 4 which feeds the fluidisation means of the first
regenerator. A larger amount of coke can thus be burned off.
If, however, the signal transmitted by sensor 32 indicates a value which is
lower than the set value, valve 30 is partially closed to reduce thermal
exchange. At the same time, the oxygen consumption in the first
regenerator is decreased and thus less coke is burned off, boosting the
catalyst temperature in the second regenerator. The temperature is thus
maintained at a substantially constant value across a range of desired
values.
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